1. Field of the Invention
The present invention relates generally to an anti-skid brake control system for an automotive vehicle. More specifically, the invention relates to an anti-skid brake control system which enhances vehicular braking performance, particularly at braking during cornering.
2. Description of the Background Art
It is well known that vehicular braking performance becomes optimum when wheel slippage becomes a certain value, e.g. 10% to 20%. Therefore, as is well known, anti-skid or anti-lock brake control is generally performed for maintaining the wheel slippage within a predetermined optimal range. For this, wheel slippage is monitored for controlling operational modes of a vehicular braking system for increasing braking pressure in an APPLICATION mode, holding braking pressure constant in a HOLD mode and decreasing braking pressure in a RELEASE mode. In vehicular anti-skid brake control, the APPLICATION mode is selected for a normal operational condition for allowing linear decelerating of the vehicle according to an increasing braking pressure. Anti-skid control is initiated upon detection of wheel deceleration during braking operation, in a greater magnitude than a preset deceleration threshold to switch the operational mode of the brake system from an APPLICATION mode to a HOLD mode. Therefore, at the initial stage of an anti-skid brake control cycle, the braking pressure is held constant at an increased pressure at which the wheel deceleration increase across the wheel deceleration threshold is obtained, in a HOLD mode. Because of the increased pressure in this HOLD mode, wheel speed further decelerates at a greater rate than deceleration of the vehicle speed. Therefore, wheel slippage which represents a ratio of the difference of the vehicle speed and the wheel speed versus the vehicle speed, becomes greater than a target speed which represents the optimal wheel speed for obtaining optimum vehicular braking characteristics. When the wheel slippage becomes greater than a wheel slippage threshold and thus the wheel speed is decreased across the target speed, the operational mode of the brake system is again switched into the RELEASE mode for decreasing the braking pressure for resuming wheel speed toward the target speed. By this, the wheel speed resumes across the target speed. Therefore, wheel acceleration increases across a present acceleration threshold. Then, the mode is again switched into the HOLD mode. Because of decreased braking pressure, the wheel speed overshoots to increase across the vehicle speed and then is decelerated to the vehicle speed. Accordingly, the wheel acceleration again decreases across the acceleration threshold. Then, the mode is switched to APPLICATION mode again.
In the anti-skid control set forth above, it becomes necessary to monitor the vehicle speed for deriving the wheel slippage. It is possible to directly measure the vehicle speed by means of an appropriate sensor, such as a doppler sensor. However, such a sensor is unacceptably expensive and therefore is not practical for use in the anti-skid control system in view of the cost. Therefore, a usual way for monitoring the vehicle speed for anti-skid control is to latch a wheel speed upon initiation of anti-skid control as vehicle speed representing data, because upon initiation of the anti-skid control where the wheel deceleration increased across the wheel deceleration threshold, it is approximately coincident with the vehicle speed. This vehicle speed representing data will be hereafter referred to as the "projected speed". Based on the latched value, vehicle speed represented data is projected utilizing a given vehicle deceleration indicative gradient which can be derived in various ways.
One of the typical processes for deriving the projected vehicle speed representative data is by selecting a highest value of wheel speed data for use as initial data in derivation of the projected vehicle speed representative data. such a strategy of selection of the wheel speed data to be used for projection of the vehicle speed representative data has been proposed in Japanese Patent Second (allowed) Publication 41-17082. During a braking operation, the selected highest value of the wheel speed data is held as an initial value of the projected vehicle speed representative data when the wheel deceleration increases across a predetermined deceleration threshold so that the vehicle speed representative data can be projected with the initial value and a given gradient of vehicle speed variation.
Such a manner of selection of the wheel speed data to be used for projecting the vehicular speed may provide an acceptably high level of precision in most occasions. However, such a manner of selection of the wheel speed data may induce a problem in derivation of the projected vehicular speed representative data in a braking operation during cornering. As can be appreciated, during cornering, the wheel speed at the outer wheel is higher than that at the inner wheel. Therefore, if the projected vehicular speed representative value is derived on the basis of the highest wheel speed data, and if the wheel slippage criterion is derived on the basis of the projected vehicular speed representative value thus derived, erroneous detection of an occurrence of wheel slippage greater than the wheel slippage criterion can be caused due to low wheel speed at the inner wheel. Also, during high speed cornering on a high friction road, such as a dry road, since the tail of the vehicle can be forced toward the outside of the curve, the wheel speed at the rear wheels becomes higher than at the front wheels. When the wheel speed difference at the front and rear wheels becomes substantial, erroneous detection of wheel slippage in excess of the wheel slippage criterion can be caused at the front wheels for erroneously initiating anti-skid control.